The size of an advertised reduction depends entirely on the reference price it is measured from. Raising that reference before a promotion enlarges the discount without lowering the price.

The reference does the advertising

A discount claim is a comparison between two figures, and the claim can be strengthened by moving either one.

Lowering the selling price costs margin on every unit. Raising the prior price costs nothing if few units sell at it.

That asymmetry creates a persistent incentive to manage the reference figure, which is why regulation focuses on it rather than on the promotional price.

Prior price rules define what may be claimed

Consumer protection frameworks in many markets require a reference price to be one at which the item was genuinely offered for a meaningful period beforehand.

Some jurisdictions specify the lowest price applied during a defined window before the promotion, which prevents a brief increase from establishing a new reference.

The details vary considerably between markets and change over time, so the same promotional display can be compliant in one jurisdiction and not in another.

Not every pre-sale increase is manipulation

Prices also rise ahead of peak periods for ordinary reasons, including higher demand, increased supply costs and reduced availability of stock.

A product entering its strongest season may legitimately trade higher, and a subsequent reduction from that level is a genuine change.

Distinguishing the two requires the price history rather than the display, since the display shows only the two figures the seller has chosen.

Recommended prices work differently

Some comparisons are made against a manufacturer's suggested price rather than against the seller's own previous price.

Where a product is rarely sold anywhere near that suggested figure, the comparison describes a price the shopper could not realistically have paid.

Rules on this vary, and the practice is generally more constrained where the suggested price has no meaningful trading history behind it.

Price history is the only real check

A record of what an item has actually cost over preceding months answers the question that the promotional display cannot.

The useful comparison is against the item's typical price rather than against its highest, since the typical figure is what a shopper would otherwise have paid.

Without that history, the size of a reduction is uninformative, because both numbers in the comparison were selected by the party making the claim.